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When a developer uses an identifier that has the same name as a public class, such as Vector, a subsequent maintainer might be unaware that this identifier does not actually refer to java.util.Vector and might unintentionally use the custom Vector rather than the original java.util.Vector class. The custom type Vector can shadow a class name from java.util.Vector, as specified by the The Java Language Specification (JLS), §6.3.2, "Obscured Declarations" [JLS 2005], and unexpected program behavior can occur.

Well-defined import statements can resolve these issues. However, when reused name definitions are imported from other packages, use of the type-import-on-demand declaration (see the JLS, §7.5.2, "Type-Import-on-Demand Declaration" [JLS 2005]) can complicate a programmer's attempt to determine which specific definition was intended to be used. Additionally, a common practice that can lead to errors is to produce the import statements after writing the code, often via automatic inclusion of import statements by an IDE, which creates further ambiguity with respect to the names. When a custom type is found earlier than the intended type in the Java include path, no further searches are conducted. Consequently, the wrong type is silently adopted.

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class MyVector {
  //otherOther code
}

When the developer and organization control the original shadowed class, it may be preferable to change the design strategy of the original in accordance with Bloch's Effective Java [Bloch 2008], Item 16, "Prefer interfaces Interfaces to abstract classesAbstract Classes." Changing the original class into an interface would permit class MyVector to declare that it implements the hypothetical Vector interface. With this technique, client code that intended to use MyVector would remain compatible with code that uses the original implementation of Vector.

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An automated tool can easily detect reuse of the set of names representing public classes or interfaces from the Java Standard Library.

Related Guidelines

Bibliography

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